This invention generally relates to a vehicle steering system. More particularly, the present invention pertains to a rear wheel steering angle control apparatus for controlling a rear wheel steering angle relative to a front wheel steering angle.
An example of a rear wheel steering angle control apparatus is disclosed in Japanese Patent Laid-Open Publication No. 61(1986)-257368. Referring to FIG. 13, this known rear wheel steering angle control apparatus is provided with a vehicle speed detecting means 1 for detecting a vehicle speed, a steering angle ratio determining means 2, an output means 3, and a rear wheel steering mechanism 4. The steering angle ratio determining means 2 determines a steering angle ratio for controlling the rear wheels in an opposite direction relative to the front wheels (opposite-direction rear-steer) when the vehicle speed detected by the vehicle speed detecting means 1 is low and determines a steering angle ratio for controlling the rear wheels in the same direction relative to the front wheels (same-direction rear-steer) when the detected vehicle speed is high. The output means 3 outputs a control signal corresponding to the steering angle ratio determined by the steering angle ratio determining means 2. The rear wheel steering mechanism 4 sets a rear wheel steering angle to the determined steering angle ratio in response to the control signal.
The rear wheel steering angle control apparatus is further provided with a set steering angle ratio detecting means 6, a judging means 7, and a vehicle speed reduction control means 5. The set steering angle ratio detecting means 6 detects the steering angle ratio set by the rear wheel steering mechanism 4. The judgment means 7 judges whether or not the difference between the steering angle ratio determined by the steering angle ratio determining means 2 and the set steering angle ratio detected by the set steering angle ratio detecting means 6 is within a predetermined allowable range. When the difference is beyond the allowable range, the vehicle speed is reduced by the vehicle speed reduction control means 5. Therefore, an abnormal condition for setting the steering angle ratio caused due to a malfunction of the rear wheel steering mechanism 4 can be detected.
When the difference between the determined steering angle ratio and the set steering angle ratio detected by the detecting means 6 is greater than a predetermined value in a state where the abnormal condition resulting from a malfunction of the rear wheel steering mechanism 4 is detected, the vehicle speed reduction control means 5 reduces the vehicle speed.
Therefore, the vehicle can be prevented from running in unstable running directions when a vehicle driving stability is deteriorated by a malfunction of the rear wheel steering mechanism 4. Therefore, the safety performance for driving the vehicle can be improved.
However, assuming that the steering angle ratio determining means 2 malfunctions, the vehicle speed reduction control means 5 may reduce the vehicle speed, when it is not needed, in response to a wrong direction of the malfunctioning steering angle ratio determining means 2. On the other hand, the vehicle speed reduction control means 5 may not function to reduce the vehicle speed, when it is needed, due to the malfunction of the steering angle ratio determining means 2. That is, the vehicle speed control may not be performed properly. Also, when the set steering angle ratio detecting means 6 malfunctions, the same drawbacks may occur.
Further, when the rear wheel steering mechanism 4 without a neutral position return function malfunctions beyond the allowable range, the vehicle speed will be continuously reduced. Additionally, when the rear wheel steering mechanism 4 malfunctions, the vehicle speed is restrained. In such a case, the other controls associated with the vehicle speed may not be properly performed.
Another rear wheel steering angle control apparatus is disclosed in Japanese Patent Laid-Open Publication No. 3(1991)-153471. In this apparatus, the rear wheels are automatically returned to a neutral position when the rear wheel steering mechanism 4 malfunctions. In such a case, a driver may feel unexpected vehicle behavior. Although the driver is warned that the rear wheels are automatically returned to the neutral position in the case that the rear wheel steering mechanism 4 malfunctions, the other controls may not be properly performed. A further rear wheel steering angle control apparatus described in Japanese Patent Laid-Open Publication No. 7(1996)-47963 by the same applicant does not overcome these difficulties.
Additionally, in the rear wheel steering angle control apparatus disclosed in Japanese Patent Laid-Open Publication No. 3(1991)-153471, a warning lamp and a neutral position biasing means are controlled by a single driving force interrupting means signal. Therefore, the vehicle may be in a 2WS (two-wheel-steering) state when the vehicle is initially started.
Accordingly, the above disclosed rear wheel steering angle control apparatus are still susceptible of certain improvements with respect to performing a normal vehicle running without drastically varying the vehicle running speed when a system of rear wheel steering angle control signals is detected to be abnormal.
A rear wheel steering angle control apparatus includes a rear wheel steering angle determining unit, a rear wheel steering mechanism, an observing unit, a neutral position detecting unit, and an alerting determining unit. The rear wheel steering angle determining unit determines a steering angle ratio based upon a vehicle speed while the rear wheel steering mechanism outputs a control signal corresponding to the steering angle ratio and sets a rear wheel steering angle corresponding to the control signal. The observing unit detects an abnormal condition of inputting and calculating processes of the rear wheel steering angle determining unit and an abnormal condition of an output signal supplied to the rear wheel steering mechanism. The neutral position detecting unit detects whether or not the output of the rear wheel steering mechanism is within a predetermined allowable range including a neutral position. The alerting determining unit provides an alert that the output of the rear wheel steering mechanism is not within the predetermined allowable range when the observing unit detects the abnormal condition and releases the alert when the output of the rear wheel steering mechanism is within the predetermined allowable range.